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  • Title: Side-chain cleavage of C21 steroids by testicular microsomal cytochrome P-450 (17 alpha-hydroxylase/lyase): involvement of heme.
    Author: Nakajin S, Hall PF.
    Journal: J Steroid Biochem; 1983 Sep; 19(3):1345-8. PubMed ID: 6604843.
    Abstract:
    The two steps in the side-chain cleavage of C21 steroids to give C19 steroids (i.e. 17 alpha-hydroxylation and C17,20 lyase activity) were examined using a highly purified cytochrome P-450 from microsomes of neonatal pig testis to determine the photochemical action spectra for the two reactions. Photochemical action spectra, using either 4-ene (progesterone) or 5-ene (pregnenolone) substrates, showed maximal reversal of inhibition by CO with light of 451 nm. Evidently the heme of cytochrome P-450 is involved in both 17 alpha-hydroxylation and in C17,20-lyase activity as in the case of the side-chain cleavage of cholesterol. Mechanisms proposed to account for enzymatic cleavage of the alpha-ketol side-chain of C21 steroids (C17,20 lyase activity) must be consistent with these findings.
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